The present invention relates to treatment of aqueous waste streams containing minor amounts of cyanide so as to detoxify such streams. More particularly, it relates to reducing the total cyanide content of aqueous waste streams containing amounts of the order of up to 1000 parts by weight of cyanide per million parts by weight of aqueous waste (1000 ppm) to less than one ppm of cyanide.
There are many industrial sources of aqueous cyanide wastes such as electroplating, steel and coke operations and the manufacture of chemicals. Hydrogen cyanide (HCN) is produced, for example, as a by-product in the production of acrylonitrile by the catalytic reaction of propylene, oxygen and ammonia and minor amounts of this compound occur in some of the non-process aqueous effluent streams in an acrylonitrile plant such as paving run-off water and contaminated condensate, for example. Because of the high toxicity of this contaminant, such streams can only be disposed of if they meet stringent specifications for cyanide content. Generally, they must contain &lt;1 ppm cyanide. Accordingly, some treatment to reduce cyanide content to this level is mandatory.
It is known to treat fluids for the removal of cyanide by contacting them with formaldehyde under basic conditions. Such a process is described in U.S. Pat. No. 2,859,090 for removing a major portion of HCN and only a minor portion of H.sub.2 S from catalytically cracked fluids containing HCN, H.sub.2 S, NH.sub.3 and moisture. Also, treatment of aqueous wastes for removal of cyanide with formaldehyde in which formaldehyde cyanohydrin is condensed to a nontoxic polymeric material is described in the article by Shen and Nordquist entitled "Cyanide Removal From Aqueous Waste by Polymerization," Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 1, pp 70-75 (1974). However, when methods such as these are applied to streams containing minor amounts of cyanide, the rate of reaction which initially is quite rapid slows down appreciably as the cyanide level decreases, the reaction rate curve essentially leveling off at various concentrations depending upon the molar ratio of the reactants. The amount of time required for reduction to the specified level of &lt;1 ppm at economic levels of treating agents, therefore, can become impracticably lengthy. It has now been discovered that the reaction can be conducted so that the rate of reaction at the low levels of cyanide concentration can be significantly accelerated and detoxification with formaldehyde can be effected in a commercially feasible reaction time. This is accomplished by employing in the reaction with the formaldehyde a compound which will generate the bisulfite ion, HSO.sub.3 .sup.-, such as SO.sub.2, Na.sub.2 S.sub.2 O.sub.5, and the like.